Sir

We share Jens Clausen's opinion, expressed in his Commentary 'Man, machine and in between' (Nature 457, 1080–1081; 2009), that brain–machine interfaces promise many benefits and should be pursued. However, we do not agree that these technologies pose similar ethical challenges to those already addressed. Some consequences may be unprecedented.

Imagine if insights from the field of cortical prosthetics in human and non-human primates were combined with research on bodily self-consciousness in humans. Signals recorded by multi-electrodes implanted in the motor cortex can already be used to control robotic arms and legs. Cognitive cortical prosthetics will allow the use of other cortical signals and regions for prosthesis control. Several research groups are investigating indications that the conscious experience of being in a body can be experimentally manipulated. The frontal and temporoparietal signals that seem to be involved encode fundamental aspects of the self, such as where humans experience themselves to be in space and which body they identify with (O. Blanke and T. Metzinger Trends Cogn. Sci. 13, 7–13; 2009).If research on cortical prosthetics and on the bodily self were applied to humans using brain-controlled prosthetic devices, there might be no clear answer to Clausen's question: which of them is responsible for involuntary acts?

It may sound like science fiction, but if human brain regions involved in bodily self-consciousness were to be monitored and manipulated online via a machine, then not only will the boundary between user and robot become unclear, but human identity may change, as such bodily signals are crucial for the self and the 'I' of conscious experience. Such consequences differ from those outlined by Clausen for deep brain stimulation and treatment with psychoactive drugs.

New links between cognitive neuroscience, engineering, brain–machine interfacing and medicine could lead to a generation of technologies that may not just blur the limits of human and machine, but fundamentally alter an individual's sense of self. We should welcome the machine to the brain, but should proceed with caution, given that such an addition could change the criteria for self and identity.